1. Field of the Invention
The present invention relates to overhead recessed, surface, and suspended lighting fixtures or luminaires used in direct or direct indirect lighting applications and in particular to a shielding media used in the luminaires.
2. Description of the Related Art
A conventional lighting fixture 10, as shown in FIG. 1, includes a metal housing accommodated in a conventional ceiling grid 12 in which one or more fluorescent lamps are mounted. One type of conventional lighting fixture also includes a shielding assembly mounted in the housing for directing the light emitted from the fluorescent lamps in a desired fashion. A known parabolic louver shielding assembly, as shown in FIGS. 1 and 2, includes longitudinal vanes 14 having curved reflecting surfaces and transversely extending cross vanes 16 also having curved reflecting surfaces.
The longitudinal vanes 14 and cross vanes 16 of the prior art louver assembly reflect light from the curved blades of the vanes 14,16 at an angle with respect to a vertical which is no greater than a cut off angle .beta.. Therefore, the louver assembly creates a shielded zone which extends from the horizontal surface of the finished ceiling plane through an angle .alpha. and prevents light from being emitted or reflected from the fixture into the shielded zone.
The shielded zone defined by the angle .alpha. protects operators of visual display terminals by preventing the operator from viewing a reflection of a luminaire in the display terminal. Light which is emitted in the shielded zone which is defined by an angle A of about 35 degrees has the greatest chance of reflecting off a display terminal into an operator's eyes, creating glare. Therefore, it is preferred that light be reduced or eliminated from a shielded zone of 35 degrees.
The Illuminating Engineering Society of North America has set forth recommended practices (RP-24) for lighting in offices containing computer visual display terminals. In accordance with RP-24 the preferred maximum luminances for direct lighting at 55, 65 and 75 cut off angles .beta., should be 850, 350 and 175 respectively measured in candelas per square meter.
As shown in FIG. 1, each of the cross vanes 16 of the known shielding assembly 12 extends from the reflecting surface of one longitudinal vane 14 to the reflecting surface of a neighboring longitudinal vane 14. The known cross vanes 16 have cross sectional shapes as shown in FIG. 2. The side reflective surfaces of the cross vane include two lower, curved reflecting surfaces 18 and two upper, planar reflecting surfaces 20. The lower reflecting surfaces 18 are defined by a constant radius of curvature which causes light rays L1 from a given point P on the lamp 22 to be directed downward into the room through an aperture of width A in the shielding assembly. However, the light rays L2 from the same point P on the lamp 22 which have deflected off the upper portion 20 of the reflective surface impinge against and reflect off the lower reflecting surface 18 of the opposite cross vane 16. As a result the light intensity of the rays L2 is reduced by the additional reflection. In addition, the reflected light L2 may be directed at an undesirable angle into the shielded zone.
In fact, light which is reflected off a highly specular reflecting surface such as the lower and upper reflecting surfaces 18,20, loses about fourteen percent of its intensity due to each reflection. Thus, the light which is reflected twice off two reflecting surfaces, such as the reflected light L2, loses fourteen percent of its efficiency in the first reflection and then an additional fourteen percent of its efficiency in the second reflection. Therefore, in order to achieve efficient illumination, it is desirable to minimize the number of surfaces from which the light reflects. In addition, it is desirable to prevent reflection of light into the shielded zone.